1. Field of the Invention
The present invention relates to telephone paystations and more particularly to a theft inhibiting circuit operated to detect deliberate shorting of handset wires to render the internal coin relay inoperative.
2. Background Art
Most telephone paystations operate in such a manner that when coins are inserted they are held in escrow in a coin hopper until such time as an internal coin relay is activated to divert the money, if the call has been completed, to the cash vault, or in the alternative, if the call has not been completed, to a coin return chute. The internal coin relay operates in response to a coin control potential extended from the telephone central office, to which the paystation is connected.
One technique frequently employed to initiate theft of coins from a telephone paystation includes the insertion of a sharp conductive instrument (such as a nail or pin) into the armored handset cable usually employed between the telephone paystation body and the associated telephone handset. This action effectively shorts one or more of the internal conductors within the handset cable to the associated cable armor. This action then renders the coin relay inoperative, preventing the telephone central office from collecting or returning money held in escrow. When this has taken place the thief can then return to the paystation at some later time, remove the short, thus causing the accumulated money to exit the coin return. Obviously it is desirable to prevent this type of paystation theft.
Referring now to FIG. 1, the prior art arrangement is illustrated by a simplified schematic of a typical paystation. Connection is made to a telephone central office by means of three terminals designated L1, L2 and G. G is the earth ground terminal and acts as a reference point for all voltages. The chassis of the paystation, for reasons of safety, is kept at the same voltage potential as the earth ground terminal (G=0 volts). This potential is usually referred to as chassis ground. Terminals L1 and L2 are the telephone line connections which are used for transmission of voice as well as for application of the coin battery from the telephone central office which is utilized for operation of the internal coin relay shown as CR. Coin relay CR is an electro-mechanical device which diverts coins from the coin hopper to either the cash vault (collect) or the coin return chute (refund).
FIG. 2 of the prior art illustrates the normal flow for collecting coins. At the associated telephone office talk battery (typically 48 volts) is removed from terminals L1 and L2. Coin battery (typically 150 volts) is then connected to terminal L2 and terminal G. Current then flows from terminal L2 through closed coin relay contact CT and coin relay coil CR returning to the telephone central office via terminal G. The same path is used for refunding coins, as may be seen by reference to prior art FIG. 3, except that the coin battery polarity is reversed. A further understanding of coin relay details may be had by reference to U.S. Pat. Nos. 3,759,440; 3,759,441; and 4,386,690 all of which are assigned to the same assignee as the present application.
Prior art FIGS. 4 and 5 illustrate the manner in which a telephone paystation is disabled in both the collect and refund modes, respectively. As can be seen, one or more of the wires to the handset HS is shorted to the handset cable armor AC. When this takes place, it causes a direct current between the telephone circuit electronics (TC) to the telephone chassis ground CG. Since current follows the path of least resistance, coin battery current applied to terminals L2 and G bypasses coin relay CR rendering the coin relay inoperable and permitting the coins deposited to accumulate in the coin hopper.
Accordingly, it is the purpose of this disclosure to provide a circuit that is easily retrofitted to existing telephone paystations and permits the use of existing components within the paystation and handset. The present invention provides the proper DC bias to the existing transmitter except when a short to the chassis ground is detected, in which case the transmitter is then isolated from the telephone circuit electronics, allowing operation of the coin relay by the central office and thus preventing coin theft.